Modern signal acquisition instruments typically include an analog-input section for receiving signals being acquired, an analog processor such as an amplifier or filter, a digitization system for digitizing processed analog signals, and a memory for storing the digitized signals. For example, U.S. Pat. No. 5,986,637, which issued to Etheridge et al. on Nov. 16, 1999, describes a high speed digital storage oscilloscope (DSO) having such features.
While generally successful, modern signal acquisition instruments can have problems in some applications, e.g., when acquiring signals from switched-mode power supplies, in locations with significant ground loops, or when small signals ride on large voltages. In such applications isolating the analog input stage so that it can utilize a user's ground can be beneficial. However, AC line-driven signal acquisition instruments typically must be electrically grounded relative to input AC power lines for safety and to comply with applicable electrical codes. Thus a conflict can exist between acquiring signals referenced to a user's ground and transferring the acquired information to the remainder of the signal acquisition instrument.
One approach to transferring information acquired by an isolated input stage to the remainder of an AC powered system is to use optical, capacitive, and/or inductive coupling. While such coupling can transfer analog information across grounds, this approach has problems because the gain-bandwidth product of the coupler often must be high to maintain linearity, because feedback mechanisms are generally unreliable, and because data quality is problematic. Another approach is to use optical, capacitive, and/or inductive coupling to couple digitized signals from logic referenced to the user's ground to logic referenced to the instrument's ground. However, this approach is relatively costly and complex and can require a significant amount of power.
Therefore, a new technique of coupling information gathered by an isolated input stage that is referenced to a user's ground to the remaining instrumentation that is referenced to instrument's ground would be beneficial.